Caption: NERVE IMPULSE, DRAWING Transmission of a nerve impulse from neuron A to neuron B Illustration of the transmission of a nerve impulse from a synapse on neuron A to a dendritic spine on neuron B. Vesicles liberate excitatory neurotransmitters (purple triangles) which open the sodium channels and enable Na+ ions (red spheres) to penetrate a dendritic spine on neuron B, creating an electric current which travels along the latter neuron's axon to its synapses.